For the purpose of braking rapid transit vehicles three types of braking efforts are generally utilized. The first is mechanical friction braking effort, the second is dynamic braking effort, and the third is emergency spring braking effort. The first two of these are used to control the vehicle speed during normal running of the vehicle and the third is used for emergency stops. Dynamic braking effort depends upon the kinetic energy stored in the vehicle, and employs the propulsion motors to generate electrical energy that is dissipated in provided resistors as a function of the current that flows in the resistors. At higher vehicle speeds, since the back EMF of the motors is high, more effective dynamic braking can be provided than at lower speeds. Therefore, when dynamic braking is to be provided, it is usually desirable first to apply more of the dynamic braking at higher speed and as the vehicle slows down then to apply more of the friction braking.
In the prior art practice of applying dynamic braking one problem was to control the dynamic braking effort as desired since an accurate feedback determination of the actual dynamic braking effort was not readily obtainable. In addition, it was desired to provide blending between the mechanical friction braking and the dynamic braking, such that as the dynamic braking effort decreased as a function of the lower vehicle speeds the mechanical friction braking effort built up as necessary to provide the desired total braking effort for the vehicle. As the dynamic brakes fade out, the friction brakes should come in such that the total braking effort is controlled as desired by the operator or the automatic train operation control apparatus. The friction brakes can have a significant time delay as compared to the dynamic brakes, so it is difficult to provide smooth blending such that the vehicle passengers would not sense a variation in the vehicle movement caused by this blending effort.
It is known in the prior art to provide a dynamic brake effort determination apparatus which responds to vehicle speed and motor armature current for providing some indication of the actual dynamic brake effort. Such apparatus has been provided in relation to transit vehicles as described in a published article entitled "Passenger Transfer System Will Take The Long Walk Out Of Air Travel" which appeared in the Westinghouse Engineer for January 1969 at pages 9 through 15.
A general discussion of the control of the power or braking operation of a transit vehicle in response to a train line P signal and a power or brake selection mode signal, as well as the blending of the mechanical and dynamic braking efforts, is provided in an article entitled "Propulsion Control For Passenger Trains Provides High Speed Service" that was published in the September 1970 Westinghouse Engineer at pages 143 to 149.